1. Field of the Invention
The invention relates to apparatus and methods for treating spinal disorders. More particularly, the invention relates to spinal fixation systems.
2. Summary of the Related Art
The human spine is a system of articulated vertebral segments with tissues including vertebrae, intervertebral discs, facet joints, ligaments, and muscles. The human spine generally includes 24 vertebrae and the sacrum. These 24 vertebrae are designated from the head to the pelvis (cervical, thoracic, lumbar, and sacral). There are 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae (although this number may vary from 4 to 6 lumbar vertebrae in some humans), and 4 sacral vertebrae. The spine generally includes 25 articulations; each bone articulates with the one above and below. The superior C1 vertebra articulates with the skull and the inferior L5 vertebra articulates with the sacrum. With two exceptions, articulations between the vertebrae are through intervertebral discs and bilateral facet joints. The exceptions are the occipital-C1 and C1-C2 articulations. In addition to the vertebral discs and facet joints, other structural elements of the vertebrae include ligaments which connect and allow constrained mobility of the vertebrae, and musculature attachments through tendons to fixation points on the vertebrae to allow motion and maintain stability. The spine functions mechanically to protect the neurological elements of the spinal cord, to bear load and maintain posture, and to allow motion of the trunk and neck. Failure of any structural components of the spine as a result of disease or injury may lead to loss of the mechanical integrity of the spine, which may lead to neurological injury, mechanical instability, and deformity.
Loss of mechanical integrity of the spine may result from congenital or developmental abnormality, metabolic disorder, tumor, infection, trauma, arthritis, and degenerative disc disease or injury to any of the functional units of the spine, including vertebrae, intervertebral discs, facet joints, ligaments, and muscles. Ultimately, this can lead to pain, loss of function, and/or neurological impairment.
One of the main treatment modalities for loss of mechanical integrity of the spine has been the use of spinal fixation systems. These systems function to restore the mechanical integrity of the spine, by improving spine stability and correcting deformity.
A typical spinal fixation system includes 2 primary components: bone anchors and structural members. Bone anchors allow mechanical connection to vertebrae and may include, but are not limited to, such fixation means as screws, hooks, wires, and clips. Structural members allow interconnection between the bone anchors and they include, but are not limited to, such objects as rods or plates. The strategies for correcting mechanical instability and spinal deformity are varied, but typically allow for multiple points of fixation to the spine above and below the unstable segments or areas of deformity. Structural members are attached to these multiple points of spine fixation to the spine, providing mechanical stability and/or correction of deformity by supporting load and transmitting corrective forces and moments.
The bone anchors are fixed to the structural members using a variety of mechanical mechanisms. Early constructs were generally limited to bone anchor fixation points at the ends of the structural member. These bone anchors commonly used fixed capturing on the structural members by passing the structural member though the bone anchor and fixing the anchor to the spine while captured. However, these constructs were limited in their ability to correct deformity as they were limited by the amount of force and moment, which could be imparted to the spine though a single bone anchor at the end of a structural member. This limitation gave rise to segmental fixation systems, which allow larger corrective forces and moments to be applied to the spine because of multiple points of spine fixation through the use of multiple bone anchors. The attachment of the structural member to multiple bone anchors has required a mechanical mechanism for attaching the bone anchors to the structural member after placement of the bone anchors. This has all but eliminated fixed capture bone anchors.
Current systems use dynamic capture mechanisms for attaching the structural member to the bone anchors. A variety of mechanisms have been disclosed where dynamic capture has been based on the principle of the screw thread fixation; for example, U.S. Pat. No. 5,176,680 discloses a device for fixing a spinal rod to vertebral screws, in which a spinal rod is passed through a split ring which is positioned between the prongs of a vertebral screw having a forked head. This assembly is locked into place by a locking screw threaded between the prongs of the forked head and onto the split ring. Similarly, U.S. Pat. No. 5,545,166 discloses a spinal fixation system that includes a plurality of anchor screws, clamp assemblies, pivot blocks, clamp blocks and rods that are implanted along a patient's spine to fix two or more adjacent vertebrae relative to each other. U.S. Pat. No. 5,716,415 discloses a spinal implant having upper and lower surfaces that include a plurality of triangular-shaped teeth that extend from the side surface to the side surface for engaging the vertebrae. U.S. Pat. No. 6,869,433 discloses a polyaxial screw assembly comprising a screw having cancellous threads for insertion into the cancellous bone of a vertebra, especially through the pedicle. A spherically shaped head has a convex surface and a tool recess for receiving a hex driver or other tool. The head is received within a tubular receiver having an internal concave surface and an adjacent opening. The convex surface of the head mates with the concave surface. The opening is smaller than the head so that the screw can project out of the opening without falling out of the receiver. A pressure disk sits atop the head and has a surface of mating shape to that of the head. The receiver also has a U-shaped portion which receives an elongated rod. The rod is used to connect adjoining vertebrae. An internal nut and external nut compress the rod against the pressure disk which in turn compresses the head convex portion into the receiver concave portion and locks the angular position of the receiver with respect to the screw.
To correct spine deformity, the structural member must be attached to the bone anchors on the deformed spine or ribs. Two strategies or a combination are employed to effect correction. The first is to conform the structural member to the deformed spine and attach the bone anchors to the structural member. Then correcting the deformity by further contouring the structured member to the corrected conformation, compressing and/or distracting sequential bone anchors until the final corrected conformation of the spine is achieved. The second strategy is to set the structural member to the final corrected conformation of the spine and attach the bone anchors to the structural member, correcting the spine deformity at the time of attachment of the bone anchors. When the second strategy is employed, it requires the bone anchors to move to the structural member and to be attached to the structural member.
Currently, this second strategy of moving the bone anchors to the structural member for correction of deformity is accomplished using one of two techniques or a combination.
The first technique employs reduction instrumentation, which is not integral to the structural member or bone anchor. Reduction instrumentation applies forces to the spine via the bone anchor to move the spine and bone anchor to the structural member where it is attached once the deformity is reduced. Reduction instrumentation systems generally operate via a threaded screw-type reduction action, or a plier-like lever reduction action to bring the bone anchor to the structural member. While these reduction instrumentation systems may be used to good effect, they are space occupying, commonly cumbersome, and frequently time consuming in the confines of a surgical wound. which is already filled with the bone anchors and the structural members. Their use often leads to both increased wound size and operative time, which translate to potential increased operative morbidity given increased risks of prolonged anesthesia, bleeding, and infection.
The second technique employs bone anchors with reduction action integral to the anchor itself. The techniques of Luque, sub-laminar wire bone anchors, or Wisconsin Wiring, wire passed through the spinous process anchoring bone, where the spine is reduced to the structural member by twisting wires around the structural member, are examples of this second technique. While these techniques offer some advantages over reduction instrumentation techniques and are still used on a limited basis, they have lost favor for reasons of neurological risk associated with passing sub-laminar wires and issues concerning secure fixation to the structural member. Another example of this technique is the use of long posted pedicle screws.
Such long posted pedicle screws allow a certain degree of movement of the bone anchor to the structural member by using an extended anchor-structural member fixation mechanism to facilitate movement of the anchor to the structural member. Once the structural member is captured by the fixation nut on the long posted pedicle screw, drawing the nut down moves the anchor until it is fixed on the structural member. Practically, this requires movement of the anchor in a plane that is defined by the axis of the screw and the tangent to the structural member at the point of fixation. For a single screw this is not severely limiting, but for two or more screws, this condition gives rise to multiple planes of bone anchor movement which must remain intersected at the structural member through the reduction process, a condition which can only be met by one plane provided that the starting holes for the multiple pedicle screws define a plane that contains all the individual pedicle screw longitudinal axes. This relatively severe limitation generally requires that a single long posted screw anchor be solely loaded at the reduction of deformity, which increases the risk of failure of both the reduction and the anchor.
Thus, there is a need for a bone anchor fixation mechanism, which can serve as a reduction device, is widely applicable to a variety of bone anchors beyond long posted pedicle screws and does not impose severe restrictions on anchor placement. Such a mechanism offers significant advantages for correction of spine deformity and ease of use, which would translate into improvement in patient care.